1. Field of the Invention
This invention relates generally to an apparatus that utilizes semiconductor memory and in particular to an apparatus that utilizes semiconductor memory where the semiconductor memory has one or more permanent unrepairable defects.
2. Description of Related Art
A wide variety of devices utilize memory for storage of information. For example, most computers include a disk drive as well as semiconductor read-only and random access memories to store information. Read-only memories used in a computer include an EPROM, an EEPROM, and a flash EEPROM, that are all semiconductor memories. Random access memory includes dynamic random access memory, static random access memory and video memory, for example. In some situations, an EEPROM and a flash EEPROM memory may be considered a random access memory also. The important aspect is that all of these memories are semiconductor memories.
A semiconductor memory typically is manufactured with redundant memory that can be used to replace defective memory cells, defective memory rows, and/or defective memory columns. For example, if a column of memory is defective, the defective column is replaced by a column within the redundant memory and the memory is sold as a defect free memory. However, if a semiconductor memory has more defects than can be repaired using the redundant memory, the semiconductor is usually considered defective and discarded. Similarly, if a semiconductor memory does not include redundant memory and the semiconductor memory has a permanent defect, the semiconductor memory is most likely considered defective and discarded.
Although a discarded defective semiconductor memory may have a portion of the memory that is usable, the memory is not used. Thus, memory devices are relatively expensive simply because a usable memory can not have unrepairable permanent defects.